This section provides background information related to the present disclosure, which is not necessarily prior art.
As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers, are being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Typically, an upper portion of the plastic container defines an opening. This upper portion is commonly referred to as a finish and includes some means for engaging a cap or closure to close off the opening. In the traditional injection-stretch blow molding process, the finish remains substantially in its injection molded state while the container body is formed below the finish. The finish may include at least one thread extending radially outwardly around an annular sidewall defining a thread profile. In one application, a closure member or cap may define a complementary thread, or threads, that are adapted to cooperatively mate with the threads of the finish.
In some applications, plastic containers must withstand extreme pressures, such as when containing carbonated beverages. One common design includes providing plastic containers having multiple feet-like structures formed around the base. A typical base consists of five such feet and is commonly referred to as a petaloid base. An exemplary container including a petaloid base is described in U.S. Pat. No. 7,891,513 (“'513 patent”) titled “Container Base with Feet,” which issued on Feb. 22, 2011 and is assigned to Amcor Limited. The disclosure of U.S. Pat. No. 7,891,513 is incorporated herein by reference. Additional exemplary containers are described in the following patents, the disclosures of which are also incorporated herein by reference: U.S. Pat. No. 5,484,072 titled “Self-Standing Polyester Containers for Carbonated Beverages,” which issued on Jan. 16, 1996 and is assigned to Amcor Limited; and U.S. Pat. No. 5,529,196 titled “Carbonated Beverage Container With Footed Base Structure,” which issued on Jun. 25, 1996 and is assigned to Amcor Limited.
In some instances, conventional petaloid base designs do not have the desired performance characteristics to withstand the extreme pressures of carbonated beverages, particularly when manufactured at high speeds (i.e., greater than 1,600 containers per hour for each mold cavity). For example, some petaloid base designs experience creasing at the feet thereof caused by carbonation pressure within the container after it has been filled. Under normal conditions, a filled and capped container can experience an internal carbonation pressure in excess of 75 PSI (5 bar). Creasing can become more pronounced as customers require lighter weight containers. Prior art FIG. 10 illustrates a container 210 including feet 212 that have undesirable creasing 214.
There is thus a need for a plastic container design that has a base capable of withstanding extreme internal pressure, and exhibits significant increased resistance to breakage and creasing at the feet subsequent to filling. Furthermore, there is a need to provide a container base providing increased stability to the container when empty as well as subsequent to filling. As described herein, the teachings of this present application address these needs by, for example, providing a container base with a smooth, balanced, and stable base geometry that is able to distribute internal pressure to reduce stress in the feet of the container base.